Studies were continued on the biologic importance of circulating uridine and the relative importance of the de novo and salvage pathways for uridine nucleotide biosynthesis. The role of the liver as a source and modulator of serum uridine concentrations was investigated using an isolated perfused rat liver system. Studies were initiated using isolated rat hepatocytes to obtain biochemical information on the regulation of pyrimidine biosynthesis. Studies on the disposition of C14-uridine in rats were begun. An in vitro system was developed to maintain constant concentrations of uridine in cell cultures. Concentrations of uridine equivalent to that measured in mouse sera were found to alter the cell growth inhibitory action of PALA and to alter the flux through the de novo pyrimidine pathway. A new GC-mass spectrometric technique for uridine quantitation was developed and the flux through the de novo pyrimidine pathway was determined in cultured cells using this assay in conjunction with stable isotopes.